Ultranarrow Luminescence Lines From Single InAs Quantum Dots Grown On A GaAs Substrate

Abstract

The self-assembled quantum dots (QDs) are very interesting from technological and physical points of view Even though much effort has been devoted to the study of the QDs, the information obtained in previous reports are still incomplete In this report, we have successfully obtained ultranarrow photoluminescence (PL) lines (560 peV) originating from single lnAs QDs The PL line width increased with increasing temperature The arsenic beam equivalent pressure was 1 4x1U5 Torr Following the growth of an AIo s5Ga0 65As/GaAs/Alo s5GaO 65As quantum well as a reference for the PL measurements on a (100) n+-GaAs substrate at a nominal substrate temperature of 600°C, lnAs QDs (1 8 ML) were grown at 530°C in Stranski-Krastanov growth mode Then, an AIo 35Ga0 65As barrier layer (50 nm) and a GaAs cap layer (20 nm) were grown at 600 ‘ C Finally, lnAs QDs (1 8 ML ) were grown at the same condition as that of the embedded QDs for the AFM observation Figure 1 shows a 500x500 nm2 AFM image of the self-assembled lnAs QDs The typical density, height, and diameter of the QDs were 1x10” cm * , 1 5-3 nm, and 20-40 nm, re sw c t iv e I y ‘Figure 2 shows a microscopic PL at 10 K excited using 514 5 nm line of Ar laser with about 2 pm diameter and detected using a Iiquid-nitrogen-cooled CCD detector The energy resolution of the measurement system was estimated to be 50-60 peV The luminescence with wide energy spectrum between 1 4 and 1 9 eV was observed The many sharp luminescence lines between 1 4 eV and 1 8 eV originate from the single lnAs QDs The peaks at 1 52, 1 63, and 1 83 eV are luminescences from the GaAs substrate, the GaAs QW, and the lnAs wetting layer, respectively In order to study the behavior of the single lnAs QDs, we focused our attention on the skirt of the luminescence at about 1 76 eV, where the density of the QDs was relatively small and each luminescence line of the QDs could be distinguished Figure 3 shows excitation power dependence of the luminescence of a single QD The luminescence line width decreased with decreasing the excitation power from 0 5 to 0 002 mW and saturated at a value of 50-60 peV This result suggests that the volume of the QDs is so small that it is necessary to excite the sample at a very small power of less than 0 01 mW in order to realize a condition of low excitation intensity. The measured minimum PL line width was not limited by the QDs but by the spectral resolution of the CCD detector Excitation power was 0 01 mW The luminescence line labeled by an arrow indicates that the luminescence originates from the same single QD The decrease in peak energy with increasing the temperature probably reflects the temperature dependence of the band gap energy It is notable that the line width increased from 65 peV to 310 peV with increasing the temperature from 10 to 70 K (-7 peVIK) even for the luminescence from a single QD with S-function-like density of electronic state This result contrasts with the reports that the PL line width was constant even if the temperature was changed [I ,2] where the energy resolution of the measurement system was relatively large (1 50-400 peV) Plausible explanation of the line width increase is the broadening due to acoustic-phonon scattering [3]. The self-assembled lnAs QDs were grown by MBE